Conventionally, an ultrasonic cleaning equipment using a piezoelectric transducer comprises at least one sheet of a ceramic piezoelectric transducer, shaped in a plate having a rectangular top surface, which is attached to a cleaning bath to generate an ultrasonic wave by the action of resonant frequency or antiresonant frequency in the longitudinal direction.
However, the sound pressure generated by the conventional piezoelectric transducer is concentrated at the center thereof, thus getting lower at the end thereof. Therefore, a cleaning bath, having a plurality of the piezoelectric transducer sheets attached, may cause an uneven washing, because the joint portions of the piezoelectric transducer generate a lowered sound pressure. Therefore, one approach has been made to increase the size of the sheet of the piezoelectric transducer, which however results in raising the cost of the production.
Another approach has been made to use a low gravity glass or an acrylic resin as the material for the cleaning bath, in order to increase the energy efficiency. However, there is a difference in gravity between the piezoelectric transducer and the material such as the glass and acrylic resin, resulting in inefficient transmittance of the sound wave. Thus, it has been demanded to provide a piezoelectric transducer having a low gravity.
Further, in case of a conventional piezoelectric transducer, a vibration mode in the lateral direction interferes with the vibration mode in the thickness direction, resulting in reducing a generation force for the ultrasonic wave in the thickness direction, to the extent that a coefficient of electromechanical coupling (kt) in the thickness direction decreases into 50% or less.
Then, a composite piezoelectric transducer has been proposed to integrate a piezoelectric ceramic with an organic polymer. Such a composite piezoelectric transducer is known to give a high coefficient of the electromechanical coupling (kt), and a reduced acoustic impedance, thereby increasing a propagation efficiency of the ultrasonic wave in a solvent. However, a conventional method, such as dice and fill method, for producing the composite piezoelectric transducer is disadvantage in cost. According to the conventional method, a sintered ceramic is subjected to a surface treatment, followed by forming grooves on the ceramic by using a dicing saw, into which an organic polymer is filled and set (cured). After removing an organic polymer such as protruded by the setting, the thickness is adjusted by polishing (grinding) by means of a wrap grinder, and then, a masking is applied for forming electrodes. After forming the electrodes, the masking is removed, and subjected to a polarization treatment. Namely, the conventional method accompanies complicated processes, and is not practical in view of high costs.
The present invention is accomplished for the purpose to solve such objectives, providing a composite piezoelectric transducer having no difference in sound pressure between the center and the end portion of the piezoelectric transducer, resulting in showing an even distribution of the sound wave. The composite piezoelectric transducer of the present invention also has a high coefficient of electromechanical coupling (kt), and a reduced acoustic impedance, thereby improving a propagation efficiency of the ultrasonic wave.
The objectives of the present invention are accomplished by a composite piezoelectric transducer comprising a piezoelectric ceramic, an organic polymer, and an electrode group formed only on surfaces of the piezoelectric ceramic. The organic polymer is filled in a groove formed in the piezoelectric ceramic in a state including bubbles. The electrode group comprises a first electrode and a second electrode. The first electrode is formed on a first surface of the piezoelectric ceramic, the first surface having a margin portion. The second electrode is formed on a second surface of the piezoelectric ceramic and a side surface thereof, or on the second surface, the side surface and the first surface, the second surface being opposite to the first surface in the direction of the thickness of the piezoelectric ceramic, and the first electrode being insulated from the second electrode by the margin. The groove of the piezoelectric ceramic is formed to extend from the second surface, forming the second electrode of the piezoelectric ceramic, toward the first surface forming the first electrode, and extending in the direction perpendicular to the side surface. The groove has a depth 50% to 90% of the thickness of the piezoelectric ceramic. A ratio, between a width of a non-diced area formed by forming the groove on the piezoelectric ceramic and the depth of the groove, (that is a ratio between the width of the non-diced area and the depth of the groove, of the piezoelectric ceramic) is 0.2 to 0.7.